The formulae commonly used for computations are as follows: 5.1 M (1) T = -- ----- c^{3} 114.6 T f (2) G = -- --------- a c a = angle measured by troptometer at elastic limit, in degrees.
c = diameter of specimen, inches.
f = gauge length of specimen, inches.

_G_ = modulus of elasticity in shear across the grain, pounds per square inch.
M = moment of torsion at elastic limit, inch-pounds.
T = outer fibre torsional stress at elastic limit, pounds per square inch.
SPECIAL TESTS _Spike-pulling Test_ Spike-pulling tests apply to problems of railroad maintenance, and the results are used to compare the spike-holding powers of various woods, both untreated and treated with different preservatives, and the efficiency of various forms of spikes.
Special tests are also made in which the spike is subjected to a transverse load applied repetitively by a blow.
For details of tests and results see: Cir.

38, U.S.F.S.: Instructions to engineers of timber tests, p.26.Cir.46, U.S.F.S.: Holding force of railroad spikes in wooden ties.Bul.118, U.S.F.S,: Prolonging the life of cross-ties, pp.

37-40.
_Packing Boxes_ Special tests on the strength of packing boxes of various woods have been made by the U.S.Forest Service to determine the merits of different kinds of woods as box material with the view of substituting new kinds for the more expensive ones now in use.

The methods of tests consisted in applying a load along the diagonal of a box, an action similar to that which occurs when a box is dropped on one of its corners.